The pictures in the rocks
Ever spotted those ‘weird’ things at the northern end of Deception Bay, on the rock shelf to the east of the path between the mangroves? If you have, you may have wondered what they are – and if you haven’t you may want to check them out!
Geoffrey Redman of Redcliffe Environmental Forum unearths for readers the secrets to these ‘pictures in the rocks’.
WHAT ARE THE ‘PICTURES IN THE ROCKS’?
Technically speaking, they are called Pseudofossils i.e. “false fossils”. They take on a variety of different ‘weird’ shapes, with one even looking like a giant turtle.
This formation lays in the Nambour Basin and is all a part of the ‘Landsborough Sandstone Group’, which goes from Northern Brisbane to Coolum.
It was created/deposited in the early Jurassic period some 200 million years ago. Yep you heard it right, like Jurassic Park, but with no giant dinosaurs found…yet.
HOW DID THESE SHAPES APPEAR?
These shapes are not man made, but a natural geological process. In scientific terms, these “pictures in the rocks” of remarkable concentric and linear structures, were formed by diffusion of iron-bearing solutions through the porous sandstone, and limonite deposition along joints.
What does that REALLY mean? Here are the steps of how these were formed.
- The sand sediments were deposited in the ancient channels and flood plains of a large north-easterly flowing braided river system that was present 200 million years ago.
- A few processes, some to do with mineral growth, others with erosion, then play a part. When the sand was laid down, there were small spaces between the grains, known as ‘pore spaces’ left.
- Groundwater then filters through the sediment, carrying dissolved minerals with it. Some of these minerals are left in the pore spaces, cementing the sand together. Commonly, the cementing starts and grows from a central point and as this process continues, it forms circles called “Lisegang Rings”, and also develops egg-like nodules.
- Over time, the rock structure can change, crack, fracture, causing different shapes and straight lines.
- From here and over time, groundwater can change its chemical composition, creating different reactions as well. Various chemicals, even salt crystalising, can expand and cause different changes and shapes. “Septarian nodules” are concretions that have radiating cracks and can form a “turtleback” appearance
- Lastly, we then have erosion processes, where the whole overlying sandstone which covered the area of “the pictures” was softer, and eroded over time, exposing these shapes. The cementing minerals being more resistant than the sandstone were left over and thanks to this geological process, we now have the “pictures in the rocks”.
WANT TO CHECK THEM OUT FOR YOURSELF?
These formations can be found at the start of the Deception Bay Heritage Trail, located on the corner of Beach Rd & Joseph Cres, Deception Bay.
Be careful –leaving the walking path to see these formations can be muddy and slippery.